1. Field of the Invention
The present invention relates to image forming apparatuses which form images based on electrophotography or an electrostatic recording method.
2. Description of the Related Art
Conventionally, in an image forming apparatus based on electrophotography, for example, an electrophotography photosensitive member (photosensitive member) serving as an image-bearing member is electrostatically charged, and the electrostatically charged photosensitive member is exposed in accordance with image information to form an electrostatic image. The electrostatic image is developed with toner and is then transferred to a member to be transferred to form an image. A widely used member for transferring a toner image from a photosensitive member to a member to be transferred may be a transfer roller which is abutted with the photosensitive member directly or through the member to be transferred with application of a transfer bias.
In order to suppress occurrence of defective transfer arising from variations in electric resistance of a transfer member, control methods including determination of transfer biases have been proposed such as an ATVC control and a PTVC control method as disclosed in Japanese Patent Laid-Open Nos. 2-123385 and 5-181373. These methods may include, before execution of image formation, applying voltage to a transfer unit, measuring a value (transfer current value) of current fed to the transfer unit, and setting a voltage condition for use in the transfer unit for image formation. In other words, a transfer bias to be applied to a transfer member for image formation is controlled based on a result of measurement of information regarding an electric resistance of the transfer member before the image formation is performed.
Here, such control including determination of a transfer bias is executed in a down time (or a period when image output is disabled for performing an adjusting operation, for example) during a pre-multi-rotation period, a pre-rotation period or an interval between sheets. Based on the current information regarding electric resistance of the transfer member, a voltage value (transfer voltage value) of the transfer bias for image formation is determined. The transfer voltage value determined properly at a certain time point may not be proper gradually due to changes in ambient temperature of a main body of the image forming apparatus, changes in temperature of the transfer member, or an increase of the electric resistance value caused by energization of the transfer member, for example. It may be considered to execute an ATVC control or a PTVC control frequently or to increase the frequency of execution of control including determination of the transfer bias. For example, when a condition for a change of the electric resistance value of the transfer member is satisfied such as a case where the environment (such as temperature and humidity) in which the image forming apparatus is provided or where a predetermined number of image formed sheets are acquired, an ATVC control or a PTVC control may be executed. More specifically, it may be executed frequently during a pre-multi-rotation period or a pre-rotation period or it may be executed during a time period set in an interval between sheets by interrupting a continuous printing operation, which is then performed after the execution of control.
Such an increased frequency of execution of control including determination of a transfer bias may allow application of a transfer bias based on a current electric resistance value of a transfer member even when the electric resistance value of the transfer member changes. However, the down time may be increased as a result, which may have an influence on productivity.
Accordingly, Japanese Patent Laid-Open No. 10-207262 discloses a correction control for performing a simpler control to correct a transfer voltage value determined once by an ATVC control or a PTVC control. According to the correction control, in a case where a transfer current value detected after applying a transfer bias for a transfer voltage value determined once by an ATVC control or a PTVC control is not matched with a target current value, the transfer voltage value is corrected so as to obtain the transfer current value close to the target current value. In other words, a difference current value (current difference) between the transfer current value to be detected and the target current value is calculated, and a voltage value corresponding to the current difference is added or subtracted to or from the transfer voltage value determined by an ATVC control or a PTVC control. Such a correction control may be performed in a timing such as during a pre-rotation period or an interval between sheets in a continuous printing operation but is simpler than the control including determination of a transfer bias by performing an ATVC control or a PTVC control. Thus, the influence on productivity may be reduced as much as possible.
By the way, a transfer current value detected in such a correction control as described above may depend on the presence of toner, called fogging toner, on a surface of a photosensitive member, attached to a non-image part. Fogging toner may easily occur on a surface of a photosensitive member in a case where a developer carrier in a developing unit is driven and a developing AC bias (AC voltage component of a developing bias) is applied to the developer carrier. Normally, the transfer current value may differ between a state with application of a developing AC bias and a state without it even when an equal transfer bias is applied to a transfer voltage value. More specifically, when a predetermined transfer bias for a transfer voltage value is applied and the developing AC bias is changed from an OFF state to an ON state, the transfer current value decreases since a region of a photosensitive member having been in a developing portion reaches a transfer portion at the ON state.
FIG. 9 illustrates changes of the transfer current value based on ON/OFF states of the developing AC bias. As illustrated in FIG. 9, the developing AC bias is normally turned to an ON state in a timing immediately before an image area during a pre-rotation period. This is for the reasons below. That is, when a developer carrier is driven and the developing AC bias is turned on, the deterioration of the developer may normally advance thereby. The deterioration of a developer may desirably be suppressed by stopping a driving developer carrier and turning off a developing AC bias during a period where image formation is not performed.
An ATVC control or PTVC control as described above acquires a transfer voltage value corresponding to a target current value for image formation. For that, a transfer current value is desirably detected with a developing AC bias applied as in image formation. Accordingly, the ATVC control or PTVC control is normally performed with application of a developing AC bias. Also, the correction control is desirably performed with application of a developing AC bias for correction based on a result of detection of a transfer current value under a condition corresponding to an actual image formation.
However, a developing AC bias may not always be applied when a transfer current value is detected in the correction control. In this case, because no fogging toner exists, the detected transfer current value may be higher than that in a normal image formation with a developing AC bias applied.
Such a difference of the transfer current value detected in the correction control due to the presence or absence of fogging toner depending on the presence or absence of a developing AC bias as described above may be corrected by the following method. That is, a difference value of the transfer current value detected in the correction control assumed based on the presence or absence of a developing AC bias is defined as a predetermined set value. The set value is subtracted from the transfer current value detected without a developing AC bias applied in the correction control. The thus acquired value may be used for correcting the transfer voltage value as a value corresponding to the transfer current value with a developing AC bias applied.
However, such a difference value of a transfer current value depending on the presence or absence of a developing AC bias may largely depend on the amount of fogging toner. The amount of fogging toner may largely depend on an environment in which the image forming apparatus is provided or the level of deterioration of a developer due to repetitive executions of image formation (tolerance), for example. Generally, the amount of fogging toner is proportional to the charge amount of toner, and the amount of fogging toner increases under a high humidity/temperature with a lower charge amount or after deterioration of a developer due to repetitive executions of image formation.
Using one set value as a difference value of a transfer current value depending on the presence or absence of a developing AC bias for an amount of fogging toner largely depending on the condition may not result in proper transfer voltage value after the correction under some conditions. Thus, the transfer current value may be different from a target current and may possibly cause a defect image due to a defective transfer.